In the typical gas turbine engine, the axial flow compressor comprises a rotor surrounded by a casing. The casing is generally made in two halves, removably joined together. The rotor is made up of a plurality of stages, each comprising a rotor disc with a single row of blades located on its outer rim. The stages are joined together and to a turbine driven shaft. The casing supports a plurality of stages or annular rows of stator vanes. The stator vane stages are located between the compressor blade stages, helping to compress the air forced through the compressor and directing the air flow into the next stage of rotor blades at the proper angle to provide a smooth, even flow through the compressor.
It has long been known that the use of variable stators to control the amount of air flowing through the compressor will optimize the performance of the compressor throughout the entire operating range of the engine. To this end, selected stator vane stages (generally at the forward portion of the compressor) are provided with variable stator vanes. In the usual prior art practice, at the position of each variable stator vane the casing is provided with an opening or bore surrounded by an exterior boss. The variable stator vane, itself, has a base portion and/or a shaft portion which extends through the bore and is rotatable therein. A bearing assembly is provided in association with the bore to prevent wear of the casing and the stator vane.
Through appropriate testing, a stator schedule is developed which optimizes performance of the compressor, while maintaining acceptable stall margins, throughout the range of operation of the engine. An actuation system is provided to rotate and reposition the stator vanes of each variable stator vane stage according to the stator schedule.
In the usual practice, a circumferentially shiftable unison ring is provided for each variable stage and surrounds the casing. Each variable stator vane of each variable stage has a lever arm operatively connected to its respective unison ring. The unison rings are shifted by an appropriate drive or bell crank mechanism operated by an appropriate actuator, as is well known in the art.
The above-mentioned bushing assemblies, designed to protect each variable stator vane and the adjacent portion of the casing, are, of course, subject to wear. This can lead to metal-to-metal contact between a variable stator vane and the compressor casing. Excessive metal-to-metal contact increases friction in the variable vane system, which in turn can prevent or interfere with movement of the vanes which could result in engine stall. The bushing assembly wears as the variable stator vane is pivoted during engine operation. Some portions of the bushing assembly which are highly loaded tend to wear more than other less highly loaded portions. In prior art structures, unacceptable wear has been detected within from about 6,000 to 10,000 hours of engine operation.
Maintenance to replace the bushing assembly involves removing the compressor casing and tearing down the variable stator vane assembly. This is expensive, time consuming, and requires skilled workers.
The present invention provides a bushing assembly in a metal housing. The bushing assembly is preferably an integral, one-piece bushing, although a multi-piece bushing can be used, as will be described hereinafter. The housing/bushing assembly is bolted to the compressor casing, and can be removed and replaced without opening and removing the casing, and without removing the variable stator vane. As a result, the bushing assembly can be removed and replaced less expensively, more rapidly and requires less skill to perform.
Furthermore, the housing/bushing assembly can be axially rotated 180.degree.. As a consequence, wear on the bushing assembly ca be distributed around the circumference thereof, greatly increasing its service life. It is anticipated that bushing assembly life can be extended to about 25,000 hours.